Surge brakes utilize the differential inertia between a slowing pulling vehicle and a pulled trailer to slow the trailer in proportion to the slowing pulling vehicle. The typical surge brake is a device having a solid connection for forward forces and a spring loaded movable connection for backward forces. When the pulling vehicle is towing the trailer the solid forward connection is operative to directly connect the trailer to the pulling vehicle. When the pulling vehicle is slowing the undiminished momentum of the trailer begins to push and "run over" the pulling vehicle. This inertial force eventually builds up to overcome the spring of the spring loaded movable connection and thus moves the connection backward to operate the surge brake. The typical surge brake thus uses rather elementary properties of physics to operate a device that matches the rate of slowing of the trailer to the rate of slowing of the pulling vehicle.
One application for surge brakes is in the area of yard trailers. These trailers are four wheeled fully weight supported containers having a hinged front drawbar for connection to a pulling vehicle. The drawbar is normally connected to a steerable axle. The drawbar itself is typically perhaps six feet long with a lunette eye and hydraulic surge brake unit at its extending end. To connect the yard trailer to the pulling vehicle, the operator picks up the extending end of the drawbar and hooks the lunette eye over the pulling vehicles hitch eye. Unfortunately, due in no small part to the presence of the surge brake unit in the drawbar, the weight of the drawbar extending end can be one hundred to one hundred fifty pounds or more. The very act of lifting this weight off of the ground, let alone holding it in the air during the line up and connection process, demands significant physical strength and coordination. In addition due to the necessity of overcoming the spring of the spring loaded movable connection before the yard trailers brakes can be applied, the yard trailer is effectively without brakes unless hitched to a pulling vehicle: it is very difficult for an individual person, even with a long lever, to compress the heavy spring.